1. Field of the Invention
The invention generally relates to improved fixation of a prosthetic component to bone and, in particular, relates to an apparatus for and method of pressure injection of low viscosity bone cement into bone surfaces to improve the bone/cement interface.
2. Brief Description of the Prior Art
Total arthroplasty, or total hip replacement, is a widely accepted Orthopaedic operation used in the correction of damaged or diseased bones, and the treatment of severe arthritis. A portion of the bone is removed and replaced with suitable metal and plastic components fixed into the bone with cement. The operation facilitates the correction of deformity, the re-establishment of stability, and the relief of pain. Occasionally complications associated with the procedure arise, such as loosening of the arthroplasty components. A number of factors contribute to loosening including failure to correct a deformity, an overweight or overactive patient, an osteoarthritic joint in comparison to a rheumatoid joint and the use of a constrained prosthesis. The consequences of loosening include return of pain and deformity, the need for further reconstructive surgery with attendant technical difficulties and increased risk of sepsis and possible risk of subsequent reloosening. Failure of the fixation often originates at the interface between the prosthesis and cement, at the interface between the cement and bone or both.
Loosening of the joint arthroplasty components occurs in a number of ways. Resorption of the bone commonly occurs particularly, around the stem of a constrained prosthesis and is the result of high loads placed on the cement/bone interface leading to micromovement, resorption and gross loosening. Separation of the cement from the bone is usually due to a weak bone/cement interface and results in gross displacement of the implant. A radiolucent line is commonly seen between the cement and bone and has the following characteristics. Microscopic examination of the interface between the cement and bone indicates that the radiolucent line corresponds to a fibrous membrane which has developed at the interface. Mechanically, the fibrous membrane presents a plane of weakness between the cement and bone and a potential site for loosening. Micromovement occurs at those interfaces where a fibrous membrane is interposed. The radiolucent line, therefore, represents a site of impaired mechanical integrity.
Failure of the cement/bone interface may be due to a mechanically incompetent interface from the moment of implantation. Often, doughy cement having a viscosity at 68.degree. to 70.degree. F. of greater than 4000 poise at the 5th or 6th minute after mixing is applied to cancellous surfaces by hand and penetrates trabeculae in the bone only to limited degree, if at all. More practically it may conform to surface irregularities, but does not penetrate the bone. Poor fixation of this type leads to micromovement which in turn leads to bone resorption which results in the development of a fibrous membrane. Acrylic cements as currently used for the fixation of joint arthroplasty components to bone has inherent deficiencies in terms of establishing secure and enduring interfaces between the prosthesis and the cement and between the cement and the bone.
Bone cement is viscoelastic in its early stages. As it cures it becomes hard and in the hardened state, fastens the components in place. Several factors enhance penetration of the bone cement into bone such as; viscosity of the cement, cleanliness of the bone, and pressure applied to the cement. These factors interplay to achieve penetration of the cement in the bone. The present invention seeks to decrease the possibility of cement loosening by better pressurization during application.
There are several patents which disclose apparatus and methods for injection of bone cement and other materials.
Miller, U.S. Pat. No. 4,338,925 is directed primarily to a cement gun and pressure injection method employing a variety of tips for the cement injection gun including a femoral canal pressurizer plug and adapter. The adapter nozzle is formed of polyethylene and screws onto the end of the cement cartridge and has a tubular extension. The pressurizer is a tapered silicone rubber plug having a generally oval cross section with a central longitudinal hole therethrough to slidably receive the tubular extension. The pressurizer plug fits onto the adapter at the end of the gun to seal the proximal end of the femur after the medullary canal has been filled. Although the plugs may be furnished in a variety of sizes, the Miller device does not provide a consistently reliable seal due to the irregular shape and range of sizes of the prepared opening of the bone cavity. The tubular extension inside the hole does not provide lateral expansion of the sealing member, and in use, leakage of pressure and cement occurs.
Malcom et al, U.S. Pat. No. 4,274,163 discloses an artificial prosthesis having opening which is clamped in place over the opening of the bone to which is being secured and cement is applied through the prosthesis under pressure. The rim of the prosthesis may be provided with an O-ring seal or deformable gasket to seal against the matching bone surfaces.
Panzer, U.S. Pat. No. 3,721,229 discloses an obturator device for the injection of fluid into a body cavity through a narrow passageway. The device comprises an injection tube reciprocally movable through an obturating collar and secured thereto by an elastic sleeve of intermediate enlarged diameter disposed around the tube. The sleeve is distended to reduced diameter for movement of the tube through the passageway and, on release, to return to its natural expanded diameter to lock the tube to the passageway wall.
The present invention is distinguished over the prior art in general, and these patents in particular by an expansible, elastomeric pressure sealing plug having a generally rectangular portion, a peripheral flange surrounding the rectangular portion at one end, and a central longitudinal hole therethrough which receives a rigid wedge-shaped expander member. The expander has a central inwardly tapered bore which frictionally receives the tubular nozzle of a bone cement cartridge when the nozzle is pressed therein to form a pressure sealing relation therewith. In the relaxed condition, the sealing plug rectangular portion is frictionally received within the femoral canal opening and the flange surrounds the exterior bone surface around the opening of the femoral cavity. The expander member is inserted into the hole in the sealing plug whereby the side walls of the sealing plug are forced radially outward to seal the opening of the femoral cavity. In the expanded condition, the expander and sealing plug provide a pressure seal between the bone cavity opening and the injection gun whereby the penetrating bone cement will be injected into the bone cavity under substantial pressure and will penetrate between the trabeculae of the bone to thereby provide a bone/cement interface to securely hold the prosthetic device in place.